The invention relates to a charged particle beam device, comprising a source for emitting a charged particle beam to an object plane, particle-optical elements which are arranged along an optical axis and which comprise at least one lens for forming an image of the object plane, and a correction element for correction of spherical and/or chromatic aberration in the image of the at least one lens, said correction element comprising a multipole element for generating a magnetic and/or electric potential distribution in a correction plane extending perpendicularly to the optical axis.
A charged particle beam device of this kind is known from U.S. Pat. No. 4,414,474.
The cited Patent Specification describes a correction element for correction of spherical aberration in an electron microscope. Spherical aberration occurs because electrons entering an electron-optical lens from a point on the optical axis are deflected more at the edge of the lens than in parts of the lens which are situated nearer to the optical axis. A point situated on the optical axis is imaged by the lens as a spot having a radius r which is given by r=MCs.alpha..sup.3. Therein, Cs is the spherical aberration coefficient, M is the transverse magnification and .alpha. is the angle enclosed by the radius with respect to the optical axis between the point on the optical axis and the edge of the lens. Spherical aberration can be reduced by beam stopping; however, this is not advantageous because information of the object to be imaged is then lost. The spherical aberration coefficient can also be reduced.
A static electric or magnetic field in a charge-free space without charge current densities is described by a potential distribution satisfying Laplace's equations. In a system with cylindrical symmetry this potential distribution can be developed in a series around the optical axis, the series containing only even powers of the distance from the optical axis. The electric or magnetic fields associated with the potential distribution are found by determination of the derivatives of this distribution. When the calculation of a trajectory of a charged particle in these electric or magnetic fields takes into account only the field terms in the series development of the field around the optical axis in which powers of the distance from the optical axis amount to no more than 1, the first-order trajectory is found. When the calculation of the trajectory of the charged particle takes into account field terms with next-higher powers of the distance from the optical axis which are greater than 1, there is found the third-order trajectory which deviates from the first-order trajectory. For charged particles emanating from a point on the optical axis, the deviation in the image plane between the first-order trajectory and the third-order trajectory is called spherical aberration. For magnetic fields the coefficient of spherical aberration can be expressed in an integral along the optical axis between the object plane and the image plane. For systems with cylindrical symmetry, Scherzer has demonstrated that this integral is always negative. Therefore, spherical aberration cannot be corrected by means of cylinder-symmetrical particle-optical elements. Using a correction element comprising two sextupoles grouped around a round lens, according to the known method a third-order effect is exerted on the electron trajectory, which effect opposes the spherical aberration introduced by the cylinder-symmetrical particle-optical system. Such a correction element has the drawback that correction takes place over a comparatively long part of the optical axis and that the correction element has a length of 10 cm in this direction and occupies a comparatively large space. As a result, it is difficult to incorporate the correction element in existing particle-optical apparatus. The requirements imposed as regards the positioning of such a correction element are severe and difficult to satisfy. Moreover, as the dimensions of the correction element are greater, its susceptibility to electromagnetic disturbances also increases becauses the image information has not yet been magnified at the area of the correction element. It is an object of the invention to provide a charged particle beam device of the kind set forth which comprises a correction element having comparatively small dimensions in the direction of the optical axis.